The present invention relates to a method for racemization of optically active 2-(4-chlorophenyl)-3-methylbutyric acid which is an important intermediate for synthesizing a group of .alpha.-substituted phenylacetic esters having an insecticidal activity.
The inventors previously have found a group of .alpha.-substituted phenylacetic esters which are very different in chemical structure from conventional insecticides but have a strong insecticidal and miticidal activities on various kinds of harmful insects, as disclosed in U.S. Pat. Nos. 3,996,244 and 4,016,179 and U.S. patent application Ser. Nos. 557,694 (filed Mar. 12, 1975), now U.S. Pat. Nos. 4,058,622, 596,332 (filed July 16, 1975), 596,333 (filed July 16, 1975), now U.S. Pat. Nos. 4,016,179, 596,334 (filed July 16, 1975), now U.S. Pat. Nos. 4,062,968, 596,335 (filed July 16, 1975), now U.S. Pat. Nos. 4,039,680, and 596,336 (filed July 16, 1975), now U.S. Pat. No. 4,031,235.
The inventors have resolved the .alpha.-substituted phenylacetic acid which is the acid moiety of the esters into the optically active (+)-carboxylic acid isomer and (-)-carboxylic acid isomer, and examined the insecticidal and miticidal activities of the esters of each isomer. As a result, they have found that the esters of the (-)-carboxylic acid isomer have little or no practical activity, while those of the (+)-carboxylic acid isomer exhibit activity about two times as great as that of the racemate. The process developed by the inventors for optical resolution of this series of .alpha.-substituted phenylacetic acids is described in Japanese Patent Application (OPI) Nos. 25,544/1975 and 106,935/1975.
The inventors also studied a method for racemization of the (-)-carboxylic acid isomer obtained as a result of the optical resolution described above, for the purpose of finding an effective use for the (-)-isomer.